Luís, cannot this be a bit simplified for :cygwin?
I've always have to do this.
"libsndfile.so.1" can be automatically translated to "cygsndfile-1.dll".
Unfortunately :cygwin is no define-foreign-library target, just :unix.

nlisp-core.lisp:
fixnum and double-float just name clisp types, no classes yet. This is
just for cmucl/sbcl so far. CLHS does not require those.
The clisp numeric classes are listed in the impnotes Chapter 4.4.
(NUMBER, COMPLEX, REAL, FLOAT, RATIONAL, RATIO, INTEGER)